Field of the Invention
The present invention relates to a method of calibrating an inertial unit for mounting on a vehicle. The vehicle may be a terrestrial vehicle, an aircraft, or a ship, for which navigation requires knowledge of position, speed, and attitude (heading, roll, and pitching). The invention relates more particularly to inertial units having axisymmetric vibratory gyros.
Description of Related Art
A modern inertial navigation unit generally has an inertial core. The inertial core comprises inertial sensors such as gyros and accelerometers arranged on the axes of the measurement reference frame. The gyros measure angular rotations of the measurement reference frame relative to an inertial frame of reference and they provide the attitude of the vehicle in the geographical frame of reference after applying correction terms that seek, by way of example, to compensate for the rotation of the earth and the movement of the carrier relative to the earth. The accelerometers measure accelerations as projected into the geographical frame of reference, which accelerations are then integrated a first time to provide speed, and a second time to provide position. The accuracy of an inertial navigation unit depends directly on the errors of the inertial sensors, and more precisely on the projection of these errors onto the local geographic reference frame, and when performing inertial navigation over a long duration, position errors depend mainly on the accuracy of the gyros. The accuracy of the gyros is affected by drift errors (offset in the origin of the measured speed of rotation), by angle coding errors (disturbance of the measured vibration angle), by scale factor errors (a scale factor error is an error concerning a coefficient used for multiplying the measured speed of rotation), and by axis setting errors (orthogonality errors between the measurement axes).
In inertial cores, it is known in particular to make use of vibratory resonator gyros. Vibratory gyros are axisymmetric and make use of the Coriolis effect (also known as “Coriolis vibratory gyroscopes (CVG)), e.g. having a hemispherical resonator (known as a “hemispherical resonance gyroscopes” (HRG)), and more generally they are said to be type I as in the document “Type I and type II micromachined vibratory gyroscopes”, by Andrei M. Shkel, pp. 586-593, IEEE/ION (“Institute of Electrical and Electronics Engineers/Institute of Navigation”) PLANS 2006, San Diego, Calif., USA. Such gyros operate either in an open loop, in which case they serve to measure an absolute angle of rotation on the basis of measuring an angle representing the position of the vibration of the resonator relative to measurement electrodes (electrical angle), or else in a closed loop, in which case they serve to measure a speed of rotation on the basis of measuring the current needed to maintain the electrical angle at a given value. The measurements supplied by such vibratory gyros may suffer from errors or instabilities that are essentially a function of the position of the vibration relative to the measurement electrodes. Such errors therefore vary depending on the position of the vibration as represented by the electrical angle.
These errors are generally determined during a calibration operation that is performed in the factory, and then they are stored so as to be taken into account when performing measurements during subsequent operation of the gyros.
Nevertheless, in certain applications, inertial units may be stored for relatively long periods of time, typically several months, prior to being installed in a vehicle where they are to be used immediately after being installed. Unfortunately, errors change during storage, thereby making it necessary to perform new calibration between installing and using inertial units.
Calibration may be performed by physically pivoting the inertial core about a plurality of non-coinciding axes. That requires the inertial core to be mounted on a multiaxis turning device prior to installing the core and the device on a vehicle.
Nevertheless, a multiaxis turning device presents overall size and weight that are not compatible with certain vehicles of small dimensions. Under such circumstances, the calibration operation is performed prior to installing the inertial core in the vehicle, which means that it is not possible to perform calibration immediately prior to using the vehicle. The validity of the calibration performed then depends amongst other things on the time that has elapsed between calibration and the vehicle being used.